Various techniques, including flow cytometry, have been employed to yield sperm populations enriched with respect to certain desired characteristics. In the livestock production industry, an ability to influence reproductive outcomes has obvious advantages. For example, gender pre-selection provides an economic benefit to the dairy industry in that pre-selecting female offspring ensures the birth of dairy cows. Similarly, the beef industry, as well as the pork industry, and other meat producers benefit from the production of males. Additionally, endangered or exotic species can be placed on accelerated breeding programs with an increased percentage of female offspring.
Previous efforts to produce commercially viable populations of sperm sorted for X-chromosome bearing sperm or Y-chromosome bearing sperm largely relied on droplet sorting in jet-in-air flow cytometers. (See e.g. U.S. Pat. No. 6,357,307; U.S. Pat. No. 5,985,216; and U.S. Pat. No. 5,135,759). However, certain drawbacks exist with these methods and devices. Even with advances in droplet flow cytometry, practical limitations still exist which hinder the number of sperm cells that can be sorted in a particular window. As such, sex-sorted artificial insemination (AI) doses are generally smaller than conventional AI doses. In bovine, for example, conventional AI doses may contain about 10 million sperm, whereas sex-sorted doses often contain about 2 million sperm. Conventional AI doses for equine and porcine are in the magnitude of hundreds of millions and billions of spermatozoa, respectively. Sex-sorted sperm, while potentially valuable, has not found widespread use in either species, because lower AI dosages generally result in lower pregnancy and birth rates. Given the large numbers of sperm required in equine and porcine, acceptable dosages have not been achieved for AI.
Sperm are time sensitive and delicate cells that lack the ability to regenerate. Accordingly, longer sorting times are injurious to sperm, as they continuously deteriorate during staining and sorting. Additionally, sperm sorted in a jet-in-air flow cytometer may be subjected to mechanical forces, torsion, stresses, strains and high powered lasers that further injure sperm. Sperm travel at velocities between about 15 m/s and about 20 m/s in the fluid stream of a jet-in-air flow cytometer. These velocities combined with the narrow stream dimensions may give rise to damaging sheering forces that can harm sperm membranes. Additionally, a high laser power is required, as sperm traveling at high velocities remain incident to the beam profile for a shorter period of time providing less of an excitation and measurement window for differentiating sperm. Finally, sperm which is ejected from a jet-in-air nozzle at 15 m/s will impact fluid in a collection container or a wall of the container at a similar velocity, presenting a further opportunity to injure sperm.